As compared with an incandescent bulb, a light emitting diode is long in service life and excellent in responsivity of light emission upon reception of electric power because of light emission principles thereof. Moreover, a light emitting diode can be used in a compact and stereoscopic manner, and therefore is less susceptible to restrictions regarding a shape as an illumination device. Further, a light emitting diode can readily realize light emission of various colors without filters or the like. Accordingly, there have been proposed various illumination devices each including such a light emitting diode serving as a light source and various light emitting diode drive devices for supplying electric power to the light emitting diode.
In recent years, particularly, a white light emitting diode has been put into practical use, and therefore uses of an illumination device including such a light emitting diode serving as a light source have been increased. In the field of vehicles, for example, a white light emitting diode is used for vehicle illumination devices such as an in-vehicle cabin illuminator, a headlight, an auxiliary lamp and a daytime running lamp.
A light emitting diode drive device for supplying electric power to a light emitting diode includes a drive circuit section that applies a predetermined direct-current voltage to a light source including a plurality of light emitting diodes connected in series in order to secure a quantity of light and allows the light source to light up. Examples of such a light emitting diode drive device include a device that controls a drive circuit section in a PWM manner in order to stably supply desired electric current or electric power to a light source, and a device that detects an electrical abnormality of a light emitting diode (e.g., refer to Patent Document 1, Patent Document 2).
In a light emitting diode drive device that allows light emission from a light source including a plurality of light emitting diodes connected in series, it is desirable to detect a short circuit of the light emitting diode which lights up.
As shown in FIG. 9, for example, there has been proposed a light emitting diode drive device having the following structure. That is, a drive circuit section 3 receives electric power from a power supply 8, converts the electric power into a predetermined direct-current voltage, and applies the direct-current voltage to a light source 2. The light source 2 includes a plurality of light emitting diodes 1, and a plurality of voltage detection circuit sections 4′ each detect a voltage across the relevant light emitting diode 1. An abnormality detection circuit section 5′ detects an abnormality of each light emitting diode 1, based on the voltage detected by the voltage detection circuit section 4′. Moreover, there has also been proposed a light emitting diode drive device having the following structure. That is, a plurality of light emitting diodes are divided into several sets, and an abnormality detection circuit section detects a voltage across each set.
The light emitting diode drive device including the abnormality detection circuit section described above can widely detect electrical abnormalities such as a short circuit and a break of the light emitting diode. However, the light emitting diode drive device needs to detect the voltage at each of the plurality of light emitting diodes. Consequently, the light emitting diode drive device requires the voltage detection circuit sections 4′ which are equal in number to the plurality of light emitting diodes. This structure complicates a circuit configuration.    Patent Document 1: Japanese Unexamined Patent Publication No. Hei 9-288456    Patent Document 2: Japanese Unexamined Patent Publication No. 2006-210835